Sulfur dioxide and/or sulfur dioxide hydrate absorbent

ABSTRACT

Disclosed is an sorbent for removing sulfur dioxide (SO 2 ) contained in combustion flue gases or in the atmosphere by using a diamine-based ionic liquid or a diamine compound supported by a polymer resin. To be specific, the present invention relates to a method of using a tertiary diamine compound immobilized on a polymer surface as a SO 2  sorbent and also relates to a novel sorbent for absorbing or adsorbing a sulfur dioxide hydrate (SO 2 .H 2 O) formed by a bond between SO 2  and water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of Korean Patent Application No. 10-2013-0034594 filed Mar. 29, 2013, and Korean Patent Application No. 10-2013-0035431 filed Apr. 1, 2013, with the Korean Intellectual Property Office and the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an absorbent or adsorbent for removing sulfur dioxide (SO₂) and a sulfur dioxide hydrate (SO₂.H₂O) using a water-immiscible diamine-based ionic liquid or a tertiary amine compound supported by a polymer resin, and more particularly, to a method of using a quaternary amine-based hydrophobic ionic liquid having a tertiary amine group or an ionic liquid supported by a resin as a SO₂ sorbent and/or a SO₂.H₂O sorbent and a process therefor.

BACKGROUND ART

For desulfurizing a power plant flue gas, there has been known a method of using lime (CaO) or limestone (CaCO₃). The use of these calcium compounds is advantageous in terms of cost but they entail a large amount of contaminated water as well as a low-value plaster as a product.

Amine compounds such as triethanolamine, monoethanolamine, diethanolamine, etc. also have been used as a SO₂ absorbent. However, a high regeneration temperature caused by a strong chemical bond between the amine and the sulfur dioxide results in irreversible decomposition of the amine. Furthermore, due to the vapor pressure of amine absorbents, regenerated SO₂ gas is frequently contaminated by the amine absorbents. Recently, as disclosed in U.S. Pat. No. 6,849,774 B2, U.S. Pat. No. 6,623,659 B2, and U.S. Patent Application Publication No. 2008/0146849 A1, methods of using an ionic liquid, which is non-volatile and has high thermal and chemical stability, have been attempted to overcome the disadvantages in the use of the conventional absorbents.

According to Angew. Chem., Int. Ed. (2004, 43, 2415-2417), 1,1,3,3-tetramethylguanidinium lactate ([TMG]L) could absorb 0.978 mol of SO₂ per mol of ionic liquid. Similarly, according to Korean Patent Application No. 10-2007-0034777, [BMIm]Cl could capture 1.68 mol per mol IL, while [EMIm]EtSO₄ could absorb 0.92 mol SO₂ per mol of ionic liquid.

Recently, in Energy Environ. Sci., 4, 1802-1806 (2011) and Chem. Commun. 2633-2635 (2012), as a method for increasing SO₂ absorption capacity, ionic liquids having an ether functional cation and a tetrazolate anion are reported, respectively. These ionic liquids could capture more than 5 mol of SO₂ per mol of ionic liquid due to the presence of Lewis basic sites which could interact with Lewis acidic SO₂

Meanwhile, flue gas contains water in the amount of 5 to 15% ionic liquid. In the presence of water, SO₂ exists as an hydrated form, SO₂.H₂O. The hydrated SO₂ is a strong acid SO₂.H₂O as described below. SO₂.H₂O⇄HSO₃ ⁻+H⁺ pKa=1.8

SO₂.H₂O is much different from SO₂ in terms of chemical property. That is, SO₂.H₂ is not a Lewis acid but Bransted acid. Therefore, when SO₂.H₂O solution is treated with amine, an ammonium bisulfite salt is produced. If the basicity of amine is high, regeneration of an absorbent is very difficult. Actually, J. Haz. Mat. 229-230 (2012) 398 describes a method for absorbing SO₂ by using an amino acid type absorbent dissovled in water and regenerating the absorbent. In the present disclosure, regeneration of the absorbent occurs at the temperature of 150° C. in which all water is evaporated together, thereby increasing regeneration energy.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to provide a novel ionic liquid compound capable of absorbing SO₂ in an anhydrous condition and in a humid condition by using a SO₂ absorbent which does not mix with water and also capable of being easily separated from water before and after absorption since it does not mix with water.

An exemplary embodiment of the present invention uses a quaternary amine-structured compound having a tertiary amine group supported by a hydrophobic ionic liquid or a polymer to provide a SO₂ and SO₂.H₂O absorbent.

The hydrophobic ionic liquid has a structure as expressed by [Formula 1] and a solid system has a structure as expressed by [Formula 2].

In Formula 1, Y⁻ represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, or PF₆—, and R₁ to R₅ respectively represent a C₁-C₆ alkyl group.

In Formula 2, X⁻ represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, PF₆—, CH₃SO₃—, or Cl—, and R₁ to R₄ respectively represent a C₁-C₆ alkyl group.

Another exemplary embodiment of the present invention provides a method for absorbing SO₂ or SO₂H₂O comprising absorbing SO₂ or SO₂.H₂O using a quaternary amine-structured compound having a tertiary amine group supported by an ionic liquid which does not mix with water or a resin and expressed by Formula 1 and Formula 2.

In Formula 1, Y⁻ represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, or PF₆—, and R₁ to R₅ respectively represent a C₁-C₆ alkyl group.

In Formula 2, X⁻ represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, PF₆—, CH₃SO₃—, or Cl—, and R₁ to R₄ respectively represent a C₁-C₆ alkyl group.

If such an absorbent is used, SO₂ and water can be separated from a flue gas through a process configured as illustrated in FIG. 2.

According to exemplary embodiments of the present invention, a SO₂ absorbent of the present invention has a high SO₂ absorption capacity, and since the SO₂ absorbent does not mix with water, it is possible to remove SO₂ from a flue gas even in the presence of water. After SO2 absorption, the absorbent could be separated from the water and regenerated in the absence of water, which could save the energy for water evaporation. Further, it is easier to remove the absorbed SO₂, and even if repeatedly used, the absorbent can maintain an absorption capacity substantially the same as the initial level without loss of the absorbent. Furthermore, the SO₂ absorbent has an absorption power with pure SO₂ or hydrates of SO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary view of a SO₂ absorption/desorption apparatus which can use a SO₂ and/or SO₂.H₂O absorbent of the present invention.

FIG. 2 is a process diagram applied to a flue gas desulfurization process. In the case of a liquid absorbent, a temperature of an absorbent in an absorption tower is maintained low, and a high-temperature flue gas introduced is cooled, whereby water in the flue gas is condensed simultaneously while SO₂ present in the flue gas is removed by the absorbent. Before and after the SO₂ is absorbed, the absorbent does not mix with the water, and thus a phase separation can be obtained. Then, the water is discharged and the SO₂ absorbent is regenerated in a regeneration tower and introduced again into the absorption tower. In the case of a solid absorbent, a temperature of the absorption tower is maintained to be lower than that of a flue gas introduced in the same manner as the liquid absorbent, whereby water in the flue gas is removed simultaneously while SO₂ in the flue gas is separated. Then, the accumulated water is separated, and if a SO₂ absorption capacity of the absorbent is saturated, the absorbent is regenerated by performing a process at high temperature.

FIG. 3 is a graph of the absorption/desorption for Example 15.

FIG. 4 is a graph of the absorption/desorption for Example 45.

DETAILED DESCRIPTION

As a result of close examination of problems of conventional amine-based absorbent and an absorption mechanism of sulfur dioxide, the present inventors found that SO₂ can be collected in an anhydrous condition or in a hydrous condition. SO₂ can be collected selectively under the condition of an excessive amount of CO₂ if an ionic liquid composed of a diamine-based cation and a hydrophobic anion or a polymer resin having diamine-based cation is used. More specifically, if an ionic liquid which can be prepared by an ion exchange reaction between an intermediate having a structure expressed by Formula 5 synthesized by using an amine compound having a structure expressed by [Formula 3] and alkyl halide having a structure expressed by [Formula 4] and a salt having a structure expressed by [Formula 6] is used or a polymer rein which can be prepared by synthesizing a diamine compound having a structure expressed by Formula 3 with a resin having a structure expressed by Formula 7 is used.

Herein, R₁ to R₄ represent a C₁-C₆ alkyl group. R₅—Cl  [Formula 4]

In Formula 4, R₅ represents alkyl groups of C₁ to C₂.

MY  [Formula 6]

In Formula 6, M represents Li, K, or Na, and Y represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, or PF₆—.

According to a method for removing SO₂ from a flue gas by using an absorbent of the present invention, SO₂ can be removed in a hydrous condition as well as in an anhydrous condition. When SO₂ is removed in an anhydrous condition, a tertiary amine of the absorbent can absorb the SO₂ through a physical interaction with the SO₂. In a hydrous condition, the SO₂ first reacts with water to be in the form of SO₂.H₂O and then reacts with the tertiary amine group of the absorbent to have a structure of [R₃NH⁺][HSO₃ ⁻]. In this case, if the absorbent of the present invention is used, [R₃NH⁺][HSO₃ ⁻] is not dissolved in the water and thus can be easily separated from the water. Further, by heating the separated salt at a temperature of 80 to 100° C. or higher, the liquid absorbent can be regenerated at a relatively low temperature.

Further, if the absorbent is used, SO₂ and water in a flue gas can be removed at the same time. If the water contained in the amount of 5 to 15% in the flue gas is continuously introduced into a CO₂ absorbent, a concentration of the liquid CO₂ absorbent is changed. Further, while the liquid CO₂ absorbent is regenerated, a boiling temperature of the water is lower than a regeneration temperature, thus, a large amount of energy is consumed during a CO₂ absorption-regeneration process. Therefore, before CO₂ is removed, the flue gas is cooled to separate water from the flue gas. In this case, while the water is separated, SO₂ can also be separated by using the absorbent of the present invention.

The process for removing SO₂ using the absorbent of the present invention consumes much less energy as compared with a high-temperature stripping process required for collecting SO₂ with an amine absorbent. This is because it is much easier to remove SO₂ from an ionic liquid absorbed SO₂ through a physical interaction rather than removing SO₂ from primary and secondary amine absorbed SO₂ by a chemical bonding. In particular, tertiary amine having a quaternary amine group has a much lower basicity as compared with general tertiary amines, thereby having a lower strength of interaction with SO₂, resulting in a relatively low regeneration temperature. When removing SO₂ in water environment, [R₃NH⁺][HSO₃ ⁻] generated which is a combination of a weak base and a strong acid. Accordingly, regeneration of [R₃NH⁺][HSO₃ ⁻] to R₃N and SO₂.H₂O may occur at a relatively low temperature.

If SO₂ is absorbed by using the absorbent of the present invention, a temperature may be in a range of 20 to 80° C., and desirably, 30 to 50° C. If an absorption temperature is 20° C. or lower, SO₂ can be present in the form of a liquid, whereas if an absorption temperature is 50° C. or higher, absorption and desorption can be carried out at the same time and SO₂ absorption efficiency can be decreased. Desirably, an absorption pressure may be in a range of normal pressure to 3 atm. considering a vapor pressure (3.44 atm. at 21° C.) of the SO₂. A desorption temperature may be in a range of 50 to 150° C., and desirably, 70 to 150° C. If a desorption temperature is 50° C. or lower, a desorption rate is decreased, whereas if a desorption temperature is 150° C. or higher, an ionic liquid can be degraded. Thus, the desorption temperature needs to be maintained in the above range. When SO₂ is absorbed, an amount of SO₂ absorbed is increased with increasing SO₂ pressure and decreasing absorption temperature. However, in most flue gases desulfurization processes, SO₂ partial pressure is not high. Therefore, it is desirable to use an ionic liquid having a high SO₂ absorption power at a low pressure. Further, a method for removing SO₂.H₂O from an aqueous solution can be carried out regardless of a temperature. A desorption temperature may be appropriate in a range of 80 to 150° C.

The absorbent of the present invention will be explained in detail with reference to Examples below. However, it should be noted that the scope of the present invention is not limited thereto.

EXAMPLE 1 Synthesis of an Ionic Liquid

Tetraethylethylenediamine and butylchloride (1:1.2 molar ratio) were CH₃CN reacted at 90° C. for 24 h. After the reaction, the remaining butylchloride and solvent were removed by using a rotary evaporator. The remaining chloride-based ionic liquid was dissolved in CH₂Cl₂ and an equivalent salt Li(CF₃SO₂)N was added thereto and reacted at room temperature for 12 h. A produced mixture of N—(N′,N′-diethylamino)ethyl-N,N-diethyl-N-butylammonium bistrifluoromethylsulfonyl imide-LiCl was washed with water to remove LiCl. Then, the remaining water was removed by using MgSO₄ and the solvent was evacuated to obtained a product (refer to Reaction Formula 1).

An absorption reactor R1 of FIG. 1 was filled with 10 g of N—(N′,N′-diethylamino)ethyl-N,N-diethyl-N-butylammonium bistrifluoromethylsulfonyl imide ([Bteeda]Ntf2) as an ionic liquid, while a temperature was maintained at 30° C. SO₂ absorption experiment was carried out by flowing SO₂ at a rate of 20 cc/min using a mass flow controller. An increase in weight over purging time was measured. When there was no further increase in weight, it was determined as a SO₂ capacity at that absorption condition. In this ionic liquid, 2.5 mol of SO₂ was absorbed by 1 mol of the ionic liquid.

EXAMPLES 2 TO 6

Examples 2 to 6 were carried out in the same manner as Example 1. In order to find out the effect of a cation of an ionic liquid on absorption of SO₂, an anion was fixed as bistrifluoromethylsulfonyl imide ((CF₃SO₂)₂N) and then an amount of SO₂ absorbed was measured. A result of the measurement was as listed in Table 1.

TABLE 1 Amount of SO₂ absorbed Example Ionic liquid (IL) (gmole SO₂/gmole IL) 1

2.5 2

2.7 3

2.3 4

2.3 5

2.5 6

2.6

EXAMPLES 7 TO 10

Examples 7 to 10 were carried out in order to find out the effect of an anion in ionic liquids in the same manner as Example 2. The cation was fixed as N—(N′,N′-dimethylamino)ethyl-N,N-dimethyl-N-butylammonium ([Btmeda]+) and the results are listed in Table 2.

TABLE 2 Amount of SO₂ absorbed Example Ionic liquid (IL) (gmole SO₂/gmole IL) 7

2.7 8

2.3 9

2.3 10 

2.5

EXAMPLES 11 TO 14

By using N—(N′,N′-dimethylamino)methyl-N,N-diethyl-N-butylammonium bistrifluoromethylsulfonyl imide as an ionic liquid and fixing an absorption pressure to 1 atm. and varying an absorption temperature as listed in Table 3, SO₂ absorption capacities were measured in the same manner as Example 1. Results of the experiments were as listed in Table 3.

EXAMPLE 15

TABLE 3 Absorption Amount of SO₂ absorbed Example temperature (° C.) (gmole SO₂/gmole IL) 11 20 2.9 12 40 1.9 13 50 1.6 14 60 1.0

By fixing an absorption temperature to 30° C. and an absorption pressure to 1 atm., SO₂ absorption capacity was measured in the same manner as Example 1. Then, while a temperature of a liquid absorbent was maintained at 80° C., nitrogen was purged at 10 mL/min to remove SO₂. The absorption/desorption cycle was repeated four times in the same condition, and a tendency of absorption/desorption was as follows.

EXAMPLES 16 TO 21

The absorption of SO2 in the presence of water was measured in the same manner as Example 1, except the same amount of water was added to the ionic liquids before SO2 purging. To obtain the SO2 absorption capacity in the presence of water, SO2 absorption amount by water was subtracted from the total weight increase. The Results are as listed in Table 4.

TABLE 4 Amount of SO₂•H₂O absorbed Example Ionic liquid (IL) (gmole SO₂/gmole IL) 16

1.28 17

1.35 18

0.971.38 19

0.981.20 20

0.971.43 21

0.991.20

EXAMPLES 22 TO 26

After SO2 absorption in the presence of water using the ionic liquid of Example 20, water was decanted and the ionic liquid-SO₂ was heated to 100° C. with a nitrogen purge (10 mL/min) so as to regenerated absorbent. The absorption/desorption experiment was repeated 5 in the same condition and the results of the comparisons are as listed in Table 5.

TABLE 5 Example 22 23 24 25 26 Number of One Two Three Four Five reuse time times times times times Amount of 1.41 0.951.38 0.971.38 0.961.32 1.33 absorption (gmole SO₂/ gmole IL)

COMPARATIVE EXAMPLE 11

In the same reaction as Example 1, instead of SO₂, CO₂ was introduced for 3 h. After CO₂ purging, no increase in weight was measured, indicating no CO₂ was absorbed by the ionic liquid.

PREPARATION EXAMPLE 1 Synthesize of an Absorbent

Merrifield resin having a Cl content of 30 mmol/g was reacted with a mixture of 5 mmol of tetramethylethylendiamine in a solvent CH₃CN at 90° C. for 24 h (refer to Reaction Formula below). After the reaction, the resin was filtered and washed with CH₃CN and dried under vacuum.

After the reaction, CHN analysis was conducted and found more than 98% of benzyl chloride site in merrifield resin were substituted by amine.

EXAMPLE 27 Experiment of Absorbing SO₂

An adsorption reactor R1 of FIG. 1 was filled with 10 g of a solid synthesized in Preparation Example 1, and while a temperature of a thermostat was maintained at 30° C., an SO₂ adsorption capacity was measured. By flowing SO₂ at a rate of 20 cc/min using a mass flow controller, an increase in weight over time was monitored. When there was no further increase in weight, it was determined as a SO₂ capacity at that absorption condition and 8.05 mol of SO₂ was absorbed by 1 g of the absorbent.

EXAMPLE 28 Synthesis of Adsorbent Including Methanesulfonate Anion

A chloride-based absorbent synthesized in Preparation Example 1 was reacted with 3 equivalents of CH₃SO₃Na in water to exchange Cl⁻ with CH₃SO₃ ⁻. A solid produced after the reaction was filtered, washed with methanol and vacuum dried. CHN analysis revealed all chloride was substituted by CH₃SO₃.

EXAMPLE 29

SO₂ adsorption experiment was carried out by using an synthesized resin in Example 28 in the same manner as Example 27 It was found that 5.0 mmol of SO₂ was adsorbed by 1 g of the adsorbent.

EXAMPLES 30 TO 34

SO₂ adsorption capacities of amine-functional polymer resins having CH₃SO₃ anion prepared in the same manner as Example 28 were measured in the same manner as Example 27. Results of the measurements were listed in Table 6.

TABLE 6 Amount of SO₂ absorbed Example Ionic liquid (IL) (gmole SO₂/gmole IL) 30

4.2 31

4.8 32

4.7 33

5.1 34

5.3

EXAMPLES 35 TO 40

Examples 35 to 40 were carried out in the same manner as Preparation Example 1 and Example 28, but adsorbents were synthesized by using various amine compounds and their SO₂ adsorption capacity was measured. Results of the measurements were as listed in Table 7.

TABLE 7 Amount of SO₂ absorbed Example Ionic liquid (IL) (gmole SO₂/gmole IL) 35

4.3 36

4.2 37

3.8 38

3.8 39

6.0 40

3.9

EXAMPLES 41 TO 44

By using an absorbent synthesized in Example 28 and fixing an absorption pressure to 1 atm. and varying an absorption temperature as listed in Table 8, SO₂ adsorption capacities were measured in the same manner as Example 28. Results of the experiments were as listed in Table 8.

TABLE 8 Absorption Amount of SO₂ absorbed Example temperature (° C.) (mmole SO₂/g absorbent) 41 20 6.2 42 40 4.3 43 50 3.8 44 60 2.5

EXAMPLE 45

By fixing an adsorption temperature to 30° C. and an adsorption pressure to 1 atm. and using an adsorbent synthesized in Example 28, SO₂ adsorption capacity was measured. Then, while a temperature of the adsorbent was maintained at 80° C., nitrogen was purged at 10 mL/min to remove SO₂. The adsorption/desorption experiment was repeated 5 times in the same condition and the results of the comparisons were as follows.

EXAMPLE 46

The adsorption capacity of SO₂ using amine-functional polymer resin of Example 28 in the presence of water was measured in the same manner as Example 1, except the same amount of water was added to the resin before SO2 purging. To obtain the SO₂ adsorption capacity in the presence of water, SO₂ absorption amount by water was subtracted from the total weight increase. The amount of SO₂ adsorption by a novel compound synthesized by the method of Example 28 was measured as 2.0 mmol/g.

TABLE 4 Amount of SO₂•H₂O absorbed Example Structure of absorbent (mmole SO₂/g absorbent) 47

2.8 48

3.1 49

2.2 50

2.1 51

2.5 52

2.2

EXAMPLES 53 TO 57

After SO₂ adsorption in the presence of water using the polymer resin of Example 51, water was decanted and the resin-SO2 was heated to 80° C. under vacuum (0.1 torr) so as to regenerated absorbent. The adsorption/desorption experiment was repeated 5 times in the same condition and the results of the comparisons were as listed in Table 5.

While a temperature of a salt of SO₂.H₂O as an absorbent of Example 39 was maintained at 80° C., water and SO₂ were removed under a depressurized condition of 0.1 Torr. If the primary absorption and removal of SO₂ was ended, absorption and removal was carried out five times in the same condition. As a result, a change in adsorption capacity was observed as listed below.

Example 53 54 55 56 57 Number One Two Three Four Five of reuse time times times times times Amount of 2.1 2.2 2.0 2.2 2.1 absorption

COMPARATIVE EXAMPLE 2

In the same reaction as Example 27, instead of SO₂, CO₂ was introduced for 3 h and an amount of CO₂ absorbed was measured. There was no increase in the weight of sample, indicating the resin did not adsorb any CO₂. 

What is claimed is:
 1. A SO₂ and/or SO₂.H₂O absorbent comprising a quaternary amine-based hydrophobic ionic liquid having a tertiary amine group and expressed by Formula 1:

Wherein, in Formula 1, Y⁻ represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, or PF₆—, and R₁ to R₅ respectively represent a C₁-C₆ alkyl group.
 2. A SO₂ and/or SO₂.H₂O adsorbent comprising a product of a reaction between a polymer resin and tertiary amine expressed by Formula 2:

Wherein, in Formula 2, X⁻ represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, PF₆—, CH₃SO₃—, or Cl—, and R₁ to R₄ respectively represent a C₁-C₆ alkyl group.
 3. A method for absorbing SO₂ and/or SO₂.H₂O by using a quaternary amine-based hydrophobic ionic liquid having a tertiary amine group and expressed by Formula 1:

Wherein, in Formula 1, Y⁻ represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, or PF₆—, and R₁ to R₅ respectively represent a C₁-C₆ alkyl group.
 4. A method for adsorbing SO₂ and/or SO₂.H₂O by using a product of a reaction between a polymer resin and tertiary amine supported by a resin and expressed by Formula 2:

Wherein, in Formula 2, X⁻ represents (CF₃SO₂)₂N—, (CF₃CF₂SO₂)₂N—, CF₃SO₃—, CF₃CF₂CF₂CF₂SO₃—, PF₆—, CH₃SO₃—, or Cl—, and R₁ to R₄, respectively represent a C₁-C₆ alkyl group.
 5. The method of claim 3 or 4, wherein a SO₂ absorption temperature is 10 to 100° C.
 6. The method of claim 3 or 4, wherein a SO₂.H₂O absorption temperature is room temperature.
 7. The method of claim 3 or 4, wherein a SO₂ removal temperature is 50 to 150° C.
 8. The method of claim 3 or 4, wherein a SO₂.H₂O removal temperature is 80 to 120° C. 